Bava Metzia, Chapter 7, Mishnah 11 - the final Mishnah of the chapter. Its topic digresses somewhat from the subject of the chapter: since at the end of the previous Mishnah we learned that a watchman, as well as the depositor, are permitted to make conditions between themselves regarding the terms of their engagement and agreement, even if they do not align with the basic rules established for the liabilities of an unpaid watchman and a paid watchman, and this condition is binding - our Mishnah comes to discuss the laws of making conditions in general.
The fundamental rule - "Kol hamatneh al mah shekasuv baTorah tenao batel" - anyone who stipulates against what is written in the Torah, his condition is void:
This rule, with which our Mishnah opens immediately, states: anyone who makes a condition in a transaction or in any halachic act that takes effect, and his conditions contradict the words of the Torah - these conditions are not binding at all.
An example of this: when a man marries a woman, the act of kiddushin that makes her his wife and forbids her to the rest of the world is performed in one of the ways, by means of money - he gives her a ring and says, "Behold, you are betrothed to me with this ring." The Torah dictates that a husband is obligated to his wife, among other things, in onah - the marital rights a woman has regarding her husband. Therefore, if he says, "Behold, you are betrothed to me with this ring on the condition that I will be exempt from onah," his condition is invalid: they are married, and he is obligated in onah. He does not have the power to make a condition that contradicts Torah law.
The source of the power to make conditions:
In truth, were it not for another portion in the Torah, conditions would have no validity at all. This is, as it were, the first stage in the system: if the Torah established that these matters take effect, where does the right to attach conditions to them come from? However, a person's power to make conditions when performing legal actions is derived from the narrative and the specific wording of the portion where the tribes of Gad and Menashe were about to cross into the Land of Israel. They asked to remain on the other side of the Jordan River, and Moshe Rabbeinu told them: if you cross over armed at the front of the battle and assist your brothers in conquering the land - you will receive this land, and if not - not. This phrasing, 'if you do this - this, and if not - not,' is the basis for the very concept of making conditions; and based on the technical details of that case, the halachah established that one must, so to speak, imitate the way those conditions were formulated. This is not the place to go into the details, as we are dealing with only one Mishnah, but this is the concept.
Therefore, when a person makes conditions that do not contradict the Torah, he has the power to define how and if his legal actions will take effect. For example, a person is permitted to give a get to his wife and say: "Here is your get, you shall be divorced from me from now if I do not return within thirty days." This condition is valid, and the get does not take effect unless the condition is fulfilled, since there is no contradiction to Torah law in this stipulation.
Two extremes: a condition regarding watching and a condition regarding nezirus:
In the previous Mishnah we learned regarding the laws of watchmen: a person says to his friend, "I will pay you money to watch my dog," and the latter agrees - he is now a paid watchman. However, he goes on to stipulate: "I will watch the dog, but if something happens to it, theft or loss, I will not pay you for it at all. If you agree - fine, and if not - not." In such a case, the condition is effective, and if harm befalls the dog, the watchman will not be required to pay. The reason for this: there is no mitzvah or obligation upon the watchman to accept the guarding of the dog, which means that this is a monetary agreement between the parties, and in monetary matters it is in the hands of the parties to determine how the arrangement will operate.
At the other extreme stands the nazir. A person who says, "Behold, I am a nazir," accepts upon himself three types of restrictions: he is forbidden to become tamei to the dead, he is forbidden to cut his hair, and he is forbidden to eat and drink grape products, including wine. If he said, "Behold, I am a nazir on the condition that I will be permitted to drink wine during the days of my nezirus" - his condition is void, since the Torah forbade wine to a nazir. And the surprising law is that he is a full-fledged nazir and is forbidden to drink wine, even though he made it clear that he only agreed to accept nezirus upon himself on the condition that he could drink wine.
The reason for this is as stated above: one who says, "Behold, I am a nazir" - is a nazir, and where does he get the power to make conditions at all? The Torah only provided room for making conditions when they comply with the rules established within the framework of the condition of the children of Gad and the children of Menashe with Moshe Rabbeinu. A condition that contradicts the words of the Torah does not enter this framework at all, and it is as if he said nothing. Therefore, we erase from his words the part that included a condition contradicting the Torah, leaving him with only the commitment to be a nazir - and hence he is obligated in nezirus. This is a concept that is difficult to understand, but this is how it works, and this is the framework of our Mishnah.
It is interesting to note an extreme case in halachah: in addition to a husband's obligation in onah, he is also obligated in she'er ukesus - to provide his wife with food and clothing. The halachah rules that a distinction is made between them:
She'er and kesus (food and clothing): Even though the Torah requires them, since they are essentially monetary obligations, if the woman says, "I wish to marry you, and I accept that you will not pay for my clothing or my food, or both," the marriage is valid and he is exempt from providing them.
Onah: The condition is invalid, because this is not a monetary matter but a physical one. If he says, "I will not provide onah" - he is married, and he is obligated to provide it regardless.
A closer look at the language of the Mishnah:
"Kol hamasneh al mah shekasuv baTorah tanao batel" - If one makes conditions that contradict the words of the Torah, his legal actions are valid regardless and his conditions are void.
"Vechol tnai sheyeish maaseh bitchilaso tanao batel" - According to the Bartenura's approach, this refers to a situation involving both conditions and a physical act, such as giving the ring along with making the condition. If he stipulated the condition after the act - meaning he performed the act first and only then made the stipulation - the condition is invalid. Many derive even more from this: if one makes two statements, the second does not take effect after he has already executed the first. We will suffice with the simpler explanation: once the act is done, he can no longer stipulate conditions after the fact.
"Vechol she'efshar lo lekaymo besofo vehisnah mitchilaso tanao kayam" - A condition that can be fulfilled in the future, and was set from the outset, is a binding condition; but a condition that cannot be fulfilled in the future, meaning it is an impossible feat, is not binding.
For example: If one says, "Here is your get on the condition that you fly to the moon" - the woman receives her get, and the condition is completely ignored, because this is not a realistic thing, and it is clear that he is merely seeking to cause her unnecessary distress; and since a person cannot fly to the moon, the condition is void.
The practical halachah in these matters is complex, and goes beyond the scope of what can be expanded upon here. With this we conclude the seventh chapter, and enter the eighth chapter - the chapter of 'Hashoel'.